High frequency amplifier



Aug. l, 1933. AQBNE ElfAL HIGH FIIEQUENGY AMPLIFIER original Filed; June 17.` 1924 s sheetssmet 2- JANDREWS ATTORNEY 1 Aug. 1, 1933.` v El AQ BEANE` ET AL v v1,920,342

HIGH .FRQUENY AMPLIFIER original Filed June 17. 1924 3 Sheets-sheet 3 'INVENToRs EDwlN A. BEANIE BY www@ ZNDREWS IFS ATTORNEY Patented Aug. 1, 1933 UNITED STATES PATENT OFFICE 1,920,342 HIGH FREQUENCY AMPLIFIER Original application June 17 1924, Serial No. 720,549. Divided and this application March 1, 1932. Serial No. 596,030

13 Claims. (Cl. 179-171.)

Our invention, which is a division of application Serial No. 720,549, filed June 17, 1924, relates to circuits and apparatus for use with thermionic tubes and to the proportioning of the elements of said circuits and apparatus for securing efficiency in the amplification of high frequency electric currents, such, for example, as are encountered in radio reception. The object of our invention is to prevent sustained or otherwise objectionable oscillation, to secure stability in radio frequency amplifiers throughout a broad band or range of carrier frequencies and, at the same time, to secure efficient selective amplification without distortion and .to accomplish these objects in a. simple and effective manner.

`Sustained oscillation is due to a transfer of energy from the output to the input circuits of thermionic tubes. This transfer of energy takes place in part through the capacitative coupling between the grid and plate element within the tube, and in part through the electro-static and electro-magnetic coupling between the transformer windings, wires, and other apparatus included in the input and output circuits of the tubes. The tendency of a three element thermionic tube to become a self excited oscillator is a fundamental characteristic of this device, when used in a radio frequency` amplifier in by means of transformers. This is especially true when the thermionic tubek in question is being used for the reception of signals at relatively high frequencies such, for example, as are used at the present time for broadcasting news and entertainment.

Sustained oscillation and instability occur when the amount of energy fed to the input circuit from the output circuit is such that the total amount of energy reaching the input circuit is equal to or greater than the total energy losses in the input circuit from a-ll sources. At lower frequencies below 150 kilocycles the natural tendency of an efficiently designed, transwhich a plurality of tubes are coupled together.

which can be built up across the plate circuit Without so great a transfer of energy back to the grid circuit that the plate circuit will exercise a detrimental control over the grid circuit thereby producing distortion and instability. This limit is somewhat lower'for Weak signals than for strong ones.

Even a transformer coupled, radio frequency amplifier, such as herein described, the plate circuit impedance of which is so designed as to avoid oscillation at a given' frequency, will oscillate when the grid circuits of the amplifier are tuned to receive a signal at va certain sufliciently higher frequency, unless the inductance of the plate circuitbe lowered or some equivalent effectl secured as the frequency to be amplified is increased.

Our invention recognizes the inherent tendency toward sustained oscillationv in transformer coupled radio frequency amplifiers, especially'at relatively r,high frequencies and contemplates limiting this inherent tendency so as not only to prevent sustained oscillation at all times but also'to secure stability over a relatively broad band of frequencies at which signals are received and to which the grid circuit is tuned.

With the foregoing object in view, our invention consists in proportioning the inductance in the plate, or output circuit, of thermionic tubes, used in radio frequency amplifiers of one or' more stages, with respect to the frequency Fto be received and the various losses of the associated circuits, so that the frequency to which the plate or output circuit is resonant shall be 90 at all times sufficiently different from the frequency ofthe carrier wave being received, and to which the grid circuit of one or more tubes -are tuned, that resonance in the plate circuit lengths.

One simple embodiment by which our invention can be practiced consists in providing the primaries lof radio frequency transformers, which primaries are connectedin the plate circuitsof radio frequency amplifying tubes, with such widely spaced turns 'and so few of them 110 that the impedance across the plate circuit will be relatively low at the highest frequencies to be impressed upon the grid circuit of the amplifier tube, so intimately coupling this primary winding with the secondary Winding of the transformer that the tuning of the circuit including the secondary winding will exercise a very great influence upon the actual impedance of this primary circuit, and at the same time mounting the transformers in non-axial positions in substantially parallel planes with their turns all running in the same direction but with the plate of the first tube connected to the outer end of the primary Winding and the grid of the succeeding tube connected to the outer end of the secondary winding of the same transformer, all of the radio frequency transformers being connected in the same'way, so that their elds oppose the transfer of electro-magnetic energy from one-to the other.

Several embodiments of our invention are illustrated' in the accompanying drawings', in which Fig. l represents diagrammatically the circuits for two stages of radio frequency amplication, where the frequency to which the plate circuits are resonant is changed only automatically through the adjustment of the next succeeding grid circuit;

Fig. 2 represents diagrammatically two stages of radio frequency amplification, Where the frequency to which the plate circuits are resonant is changed bya manual adjustment of the inductances through a change in the mutual inductance between primary turns brought about by changes in the spacing of the primary turns on the radio frequency transformers, which change in spacing is brought about mechanically responsive to tuning one or more grid circuits;

Fig. 3 represents diagrammatically a circuit arrangement similar to Fig. 1, in which the inductances of one or more plate circuits are changed through the operation of a variometer mechanically connected to move with one or more of the tuning devices for the grid circuits;

Fig. 4 represents diagrammatically av circuit similar to that of Fig. 1, but in which variotransformers are used for adjusting the impedance of the plate circuits responsive to changes in .adjustment of the impedance of the grid circuits; and

Fig. 5 represents a preferred form of variotransformer,

Fig. 6 represents a form of inductance found to be suitable for use in the system shown in Fig. 1.

Figs. 7, 8, 9 represent different modifications of the coupling network between a pair of tubes of Fig. 1.

Referring specifically to Fig. 1, the thermionic tubes V, V1 and V2 are the amplifiers and detector in a two stage radio frequency amplifying circuit in which S1, S2 and S3 are the secondaries and C1, C2 and C3, the variable condensers in the grid circuits of said amplifiers. P and P1 are the primaries located in the plate circuits of V and V1 respectively, the proper connection and winding of the transformers being indicated. y

In Fig. 2, the same circuit arrangement is shown as in Fig. 1, and the same numbers designate similar parts. The primaries P and P1 are constructed so that, ltheir inductance. can be varied as the capacityof condensersCl and C2 are varied. When the shaft 7 is rotated, this is accomplished by the discs 4 and 5 moving toward the secondaries S2 and S3, by means of the threads 8 and 9 or other suitable mechanism. As the discs 4 and 5 approach the secondaries S2 and S3, the turns in the primaries P and P1 come closer together, thus increasing their mutual inductance. The condenser C1 and the disc 4 can be separated from the condenser C2 and the disc 5 and operated by sepa.- rate controls, if desired. When the connections to the transformers are made as shown in Fig. 2, it is desirable that the primary and secondary turns run in the same direction. This causes the magnetic field around S1 to oppose the transfer of electro-magnetic energy to it from P and S2 to oppose a similar transfer from P1.

Fig. 3 is similar to Fig. 2, except that a variometer 10 varies the inductance of both primaries P and P1. This variometer has a relatively small inductance and a relatively small inductance change; its rotor 11 may be mounted onthe shaft 7 which may be common to condenser C1 and C2. A metallic shield 21 carried upon the shaft '1 prevents coupling between the condensers C1 and C2. When the capacity is minimum in condensers C1 and C2, the inductance of the variometer 10 is also at its minimum. When the inductance in the primary must be kept very low, this arrangement is not as suitable as that shown in Fig. 2 as it is desirable to keep all of the inductance in the primaries where it will be effective in securing the proper coupling to the secondaries. A variable resistance 12 may be included in the antenna circuit 13, which is used only when antennae of very low resistance are employed and when the frequency being received is very high. This resistance is only varied when the antenna is changed or when it is desired to receive very high frequencies. With the ordinary antenna, it is not required except at extremely high frequencies and it is not necessary even with a loop antenna of very low resistance at frequencies not higher than 1350 kilocycles.

Fig. 4 represents a similar circuit to Figs. 2 and 3, except that disc type variometers are used for tuning the secondaries S1 and S2 and the primaries P and P1. In this case as in those shown in the other figures, the direction of the primaries and secondaries relative to the connection of their various ends should be made so that their magnetic fields will oppose any energy transferred from P1 to S2, or P' tov S1. The element 14 of S1 is stationary while 15 rotates; the element 16 of P is stationary while 17 rotates. The element 18 of S2 is stationary While 19 rotates, etc. These coils of wire may be wound upon spider web forms; one coil above and one coil below the center as shown in Fig. 5.

It will also be seen that the coils 16 and 18 may be wound on the same form and likewise 17 and 19 as shown in Fig. 5. This forms a flat or pancake type of variable transformer with a primary consisting of a very small number of turns, half on the rotor element, and half on the -turns any'desired variation of inductance in the primary relative to the variation of the inductance in the secondarymay be had. Although only two stages of amplification have been shown, it will be understood that more stages may be added if desired, or that only one stage.

may be used.

If desired, the manual means for varying the plate circuit inductance with changes of frequency to be amplified, may be applied to only one of the plate circuits, and may becarried out in one or more large steps as shown in dotted lines in Fig. 1, instead of gradually as in Figs. 2, 3 and 4.

The operation of all of the foregoing embodiments of our invention and other embodiments coming Within the scope thereof may be explained by the following theoretical consideration, which is here included because of the absence in published form of any concise state-v ment of these theories, the understanding of which is important to a thorough understanding of our invention.

The sensitiveness and eiciency of a radio frequency amplifier depends, amongA other things, upon the impedance of the plate, or output oircuit of the thermionic tubes, to the frequency of the current being received upon the input circuits of the tubes. If the output circuit is tuned to resonance at the frequency of current in the input circuit, the impedance across the output circuit increases at that frequency and the current in the primary. of the transformer put circuit increases. as 'the impedance acrossv the output circuit to the frequency of the carrier wave being received increases, it is apparent that by reducing the inductance in the output circuit, this circuit can be brought suiciently out of resonance to the frequency. of the current in the input circuit so that the voltage across the plate circuit, and therefore the energy transferred from the plate circuit to the grid circuit, will be limited, at least to such an extent that substantially less energy will be transferred to the grid circuit from the plate circuit than is ydissipated in the grid circuit. The plate circuit must be kept sufficiently away from resonance so that it is removed by a substantial distance from this critical condition if stability in the circuit Aand freedom from distortion and noises due to more or less sustained oscillation is to be secured.

It is, f course, desirable to maintain the current in the transformer primary in the plate circuit as high as possible consistent with the elimination of undesired oscillation so as to transfer the maximum energy to the secondary in the grid circuit of the succeeding tube. It is therefore desirable to operate the plate circuit as close to resonance as possible consistent with stability over the band of frequencies to be received, in

` order to` secure the maximum radio frequency relay amplification.v It will be seen that the fundamental tendency of radio frequency amplifiers to oscillate, particularly at high frequencies, presents itself as the factor hunting radio normal energy in the grid circuit. This has an frequency relay amplification at these frequencies. In order. to prevent this oscillation, losses must be voluntarily imposed in some manner upon some one or more of the associated circuits so as to prevent the tubes from becoming self excited oscillators. It is believed that the reason for the greater eciency of radio frequency amplification at the lower frequencies is that it is possible to operate a low frequency amplier with its output circuits much lcloser to resonance without producing undesired oscillations than it is with similar amplifiers when the frequencies are higher. It is one of the outstanding advantages of this invention to impose these losses which are necessary to prevent os'- cillation in the form of reactance in the plate circuit at which point and in which form the losses appear to be least objectionable.

Heretofore, as far as We are aware sustained oscillation has always been prevented and stability secured in tuned transformer coupled radio frequency amplifiers by imposing articial losses, or some equivalent effect upon the grid-circuit. This has been done in some cases by means of a variable resistance in the grid circuit and by varying the grid bias.

Another vgeneral method which is susceptible to a number of variations is to transfer energy to the grid circuit of such phase as to oppose the effect which is similar to the method previously described inasmuch as it reducesgthe effective energy in the grid circuit thereby reducing the energy in the plate circuit. Our invention diiers from all previously employed methods for preventing sustained oscillation andl securing stability yin that it prevents the transfer of energy through the grid plate capacity solely by maintaining the impedance across the plate circuit suiiciently low so that the voltage built up across this circuit vis not .great enough to cause sustained oscillation or instability even though the losses in the grid circuit are so low that the tube would immediately become a self excited oscillator if resonance, which is coincident with maximum impedance across the plat circuit was anywhere nearly approached. The output circuit of previous radio frequency ampliers have been constructed with inductances so high relative to the frequency of the signals which they were designed to receive and with so much electro-magnetic coupling to the input circuit that theywould normally oscillate violently unless prevented from doing so by the introduction of some device for producing losses or an equivalent effect in the grid circuit. In our invention the inductance in the plate circuit-is not only reduced to the critical point at which oscillation is just prevented without the introduction of artificial losses in the grid circuit and -in which condition a disturbance will throjv the circuit into more or less sustained oscillation but the inductance of the plate circuit is reduced sufciently to bring the amplifier substantially past this critical point so that -stable operation is secured over a relatively wide tendency for electro-magnetic energy to be transferred to the secondary of the preceding transformer in the grid circuit of the same tube. The transfer of electro-magnetic energy from the plate circuit to the grid circuit is further prevented by the arrangement of these circuits so that their respective magnetic fields oppose the transfer of energy from one to the other.

We believe our method for accomplishing the above object to be superior to that in which energy of reverse phase is transferred to the grid circuit as proposed by Hazeltine in Patents No. 1,450,080 and No. 1,489,228 in that it is more simple, more direct, easier to construct and control and also for other reasons among which is that by our method the original transfer of energy is prevented instead of a transfer occurring and being afterwards neutralized by a more orV less. equal and opposite electromotive force. Our method is also superior to the grid resistance or grid biasing method above referred to for" the following reasons:

If losses introduced to prevent sustained oscillation take the form of resistance, they necessarily increase the damping of the circuits, broadening their tuning and making it difficult to separate signals of slightly different frequencies. For this, and also for other reasons, it is far less objectionable to introduce any losses necessary to prevent sustained oscillation by varying the reactance rather than by'increasing resistance. We have also found that it is preferable to introduce these reactance losses inthe plate circuit. There is a larger amount of energy in the plate circuit which permits of the use of larger values for its control. Therefore, the balancing of values for accomplishing the desired result without the introduction of excessive losses is far less critical.

Although it is desirable to reduce the resistance of all circuits, employed in the amplifiers herein described, a certain amount of resistance is nevertheless unavoidable. This is not altogether objectionable as it would be very difficult, if not impossible, to secure stable operation in such an amplifier were it not for the presence of some appreciable resistance. The lower the resistance becomes, particularly of the grid circuit, the greater the variation of the plate circuit reactance required to secure stability. Also the higher the frequency of signal being received, the greater the energy transfer from the plate circuit to the grid circuit through a given coupling. It is conceivable that at very high fre-I quencies and with all resistance losses reduced to a very low point, a condition might be arrived at under which the above described method would not be adequate for securingl stability. Should such a condition be encountered, we have shown means by which it can be overcome by placing a variable resistance in the antenna circuit. It must be borne in mind, however, that this expedient should only be adopted after the method herein described for preventing oscillation has been applied as far as possible, 4so as to make necessary the introduction of only the smallest possible'amount of resistance to secure the desired degree of stability.

vWe have found experimentally, however, that.

at frequencies of from 1350 to 550 kilocycles, it

' is possible and desirable to reduce all losses, ex-

cept plate circuit reactancelosses, to thelowest .introduction of greater losses to prevent this oscillation when they are tuned to receive signals at higher frequencies than when they are tuned to lower frequencies. It has been supposed that this was due to a more advantageous inductance capacity ratio at these frequencies in the grid circuit resulting in a higher potential across the grid circuit. We have discovered, however, that the greater tendency to oscillate at the higher frequencies is not due in any large measure to the above mentioned cause but principally to the fact that, the transfer of energy from the plate circuit to the grid circuit increases as the frequency increases. There are two reasons for this: first, a greater amount of energy can be transferred through a given coupling, other conditions remaining the same, at a higher frequency than at a lower frequency, and second, when the plate circuit is slightly above resonance at a lower frequency, and the grid circuit is then tuned to receive a signal at a higher frequency without any change of the values connected in the plate circuit, the plate circuit will be closer to resonance at this higher frequency which will result in a higher voltage across it and a greater transfer of energy to the grid circuit. If a radio frequency amplifier of the type described is to be maintained in the desired state of maximum sensitivity and responsiveness to weak incoming signals, consistent with stable operation over a relatively broad band of frequencies, the impedance of the plate circuit to the higher frequencies should be reduced as the grid circuit is tuned to receive higher frequencies thus preventing an increase in the transfer of energy from` the plate circuit to the grid circuit at. the higher frequency.

This can be done in two ways; first, by reducing the physical inductance i-n the plate circuit as the frequency ofthe signal being received increases, and second, by coupling the plate circuit of one tube to the grid circuit of the succeeding tube as closely as possible, so that when the grid circuit of the succeeding tube is tuned to a higher frequency, this effect will be imposed in the greatest possible measure upon the plate circuit of the preceding tube. This latter method, while very useful, does not with the degree of coupling so far obtained prevent the tendency toward oscillation increasing slightly as the frequency increases. However, as long as the antenna resistance is fairly high, very satisfactory results can be obtained by this method. We have also found another effect which can also be made to act against the gre er tendency toward oscillation with inc'reasesin frequency. v-This can be accomplished by'a'rranging thev connections and turns in the coils contained in the output and input circuit of an amplifier tube in such a way that avery small amount of energy is transferred electromagnetically from the outputv circuit to the input circuit of such phase as to oppose the cur- ,rent in the input circuit. Now, as the energy means of opposing the increase in 'the tendency toward oscillation at higher frequencies which may be useful under certain special conditions.

There is still another method by'which the increase in the tendency to'sustained oscillation and instability as the frequency increases can be minimized or eliminated. This consists of placing a greater amount of inductance or capacity in the plate circuit than the amount required to produce resonance at the frequency being received. A point at which the plate circuit is not resonant to the frequency being re- -ceived on the grid circuit canbe thus reached by adding to the inductance in the plate circuit as well as subtracting from it. If this inductance is increased sumciently above resonance so that oscillation is prevented at the lowest frequency to be received, as shown in connection with the primary P of Fig. 7, it being understood that the circuit elements of Fig. '7 are similar to those shown in Fig. 1 between tubes V and V1, tuning the grid circuit of tube V1 to, and receiving a signal of higher frequency will now bring the plate circuit of tube V' still furtheraway from resonance than before thereby decreasing the potential across the plate circuit and tending to decrease the transfer of energy to the grid circuit. This effect would then work against the greater transfer of energy through a given coupling at higher frequencies. In this Way the two eects may be made to more or less balance one another thereby minimizing or eliminating the increase in the tendency t0- Ward oscillation as the frequency increases. There are, however, two objections of this method, first, the increase in the inductance across this circuit, and it is conceivable with a large coil that an appreciable voltage might be built up across the plate circuit of tube V due to the impedance of the inductance coil P. This lcould be minimized, as shown in Fig. 8, by using as small a coil P as practicable with a large condenser C across it. Second, this method necessitates having a large inductance in the plate circuit, as large or larger than in the grid circuit of the succeeding tube.

Unless the inductance P in the plate circuit was separated into two inductances, only one P of which was used for the transformer primary as shown in Fig..9, a step down transformer would result instead of a step up transformer with more secondary turns than primary turns. A step up transformer is furnished naturally by the type of transformer in which the inductance of the plate circuit is reduced by reducing the primary turns. This step up transformer effect is highly desirable as it considerably increases the voltage amplification over lower ratio step up transformers or other coupling means which do not provide in themselves a high voltage amplification, it also secures a high degree of selectivity.

It is an essential feature of this invention to reduce the coupling between the platecircuit and the grid circuit of the amplifier tubes which tends to produce undesired oscillation, either by the method herein described or by the reduction of the physical coupling of these circuits through the tubes, transformers and associated circuits to the lowest point possible. The more completely this couplin,I is reduced, the less reduction will be necessary in the inductance of the plate circuits to prevent undesired oscillation, and inasmuch as the reduction of plate circuit inductance also increases the plate circuit reactance losses at the frequency being received and to which the grid circuit is tuned, and as it is desirable to maintain these losses as low as possible conistent with staple operation, it will be seen that great care should be taken to avoid the reduction of plate circuit inductance beyond the amount necessary to secure stability. it is also desirable to avoid the unnecessary reduction of plate circuit inductance for the reason that it reduces the electro-magnetic coupling between the plate circuit of one tube and the grid circuit of the succeeding tube. It is desirable to maintain this coupling quite close, at least in form of this invention where the physical inductance in the plate circuit is not varied.

By connecting the proper end of the primary coil in the plate circuit of a tube in an amplifier of the type described, to the plate, with relation to the end of the secondary coil in the grid circuit of the same tube which is connected to the grid and by maintaining this proper connection in all the coils and by fixing the coils in the proper position, a condition can be secured by which the i'leld of any transformer, or variotransformer used in the amplier can be made to oppose the field of the transformer on either side of it to just the proper degree so that the fields of these transformers will not interlink and there will be no transfer of electromagnetic energy from the plate circuit to the grid circuit of the amplifier tubes. This condition of zero magnetic coupling is the desired condition and when this is attained it is only necessary to compensate for electrostatic coupling between the plate and grid circuit by reducing plate circuit impedance. We have found with flat spider web transformers four inches in diameter with sixty-three turns in the secondary and six to eight turns in the primary, that good results are obtained when the transformers are placed parallel to one another on a common axis with a separation of approximately six and onequarter inches. When all the turns on all the transformers run in the same direction, all the grids should be connected to the outside of the secondary winding and all the plates to the outside of the primary windings. lIf the coils are put at right angles to one another or if one of them is turned through 18u degrees without at the same time turning the 'other two, sustained oscillation will immediately result and can only be checked by a verygreat reduction in the number of primary turns. It is possible to also place these coils in non-axial positions in parallel planes and still maintain the desired condition, but if the coils are placed end to end in the same plane, even with a considerable spacing, oscillation will commence.

The primary turns may be wound if desired, in the opposite direction from the secondary turns. Under these circumstances the inner end of the primary windings should be connected to the plate, all other conditions remaining the same. By placing the transformers at the proper distance, lone from the other and rotating them simultaneously to more or less axial positions, the desired condition of maximum stability can be obtained. This can likewise be accuit be made such as to avoid objectionable oscillation at the highest frequency to be received. `It is also desirable to impose the least possible reactance losses in the plate circuit at the frequency of the signals being received consistent with stability. As the grid circuit is tuned to receive signals of lower frequency, the action of the succeeding grid circuit upon the plate circuit to which it is coupled will be such as to prevent in a measure the increase of the reactance losses of the plate circuit at the lower frequencies being received. This effect will be greater the closer the coupling between the primary and secondary of the transformers employed.

It is therefore'desirable to produce a transformer which shall have the closest possible coupling between the primary and secondary and at the same time maintain a low primary inductance. We secure these characteristics by of primary turns in direct contact with the secondary turns, at the same time maintaining the minimum primary inductance consistent therewith.

To take full advantage of our invention all electrical losses vexcept plate circuit losses pur-` posely imposed should be reduced as far as possible. We therefore prefer the spider web type of y coil for transformers with a minimum of Idielectric material necessary to support the wire.

" We have devised a form of spider with a greatly reduced amount of dielectric material and also an easy way of winding these transformers consisting of winding the primary and secondary turns together as if they were one wire, then cutting offfthe primary at the desired point and continuing the secondary to the outside of the transformers are so constructed that the replifier is tuned to a higher frequency (a change.

for example of from 550 to 1350 kilocycles), the

tendency to` oscillate is greater at the higher,

frequencies despite the action of the secondary circuit upon the primary circuit.

Our invention, therefore, contemplates, where desired, means for varyingthe physical inductance in, the plate circuit so that it becomes progressively lower as the amplifier is tuned to receive higher frequencies. By a slight variation of the very small inductance in the plate circuit as the grid circuit control is adjusted, the amplifier may be maintained in that desirable state justsuciently below the critical condition in the plate circuit previously referred to while the grid circuit tuning is changed over a relatively broad band of frequencies such, for example, as are now in use Vfor broadcasting music and entertainment.

This changing of the frequency to which the plate circuit is resonant by changing the physical values 'connected into the plate circuit can be accomplished either by a variable condenser or by a variable inductance either -of which can be operated by a separate control or by being connected to the controls used for tuning any or all of the grid circuits. Where this form of the invention is employed, we prefer to use a variable inductance which constitutes the primary of an air core transformer and this inductance is varied in the desired manner by connecting it either directly or through a proportioning mechanism to the control which operlates one or more tuning devices used for tuning one or more of the grid circuits of the amplifier tubes.

What we claim is:

1. A radio frequency coupling system having input terminals and comprising a main resonant circuit including as elements a coil and a condenser, at least one of which is adjustable to tunethe coupling system over a range in frequency, a circuit inductively coupled to said resonant circuit including a fixed self-induct.` ance having a resonant frequency lower, but

not greatly lower, than the lowest frequencywithin said range and an electron discharge tube having its output electrodes coupled across opposite terminals of said fixed self-inductance.

2. In a radio amplifier stage including a vacuum tube having cathode, grid and anode electrodes, a coupling system which comprises' a main resonant circuit including as elementsthereof an inductance coil and a condenser, at least one of which is adjustable to tune the coupling system over a range in frequency, the cathode and `grid of said tube being connected across opposite terminals of said coil and condenser, a circuit coupled to said resonant circuit including a fixed inductance coil having a resonant frequency lower, but not greatly lower. than the lowest frequency within said range.

3. .In a radio amplier `stage including an electron discharge tube having a cathode, a'grid and an anode, a coupling system which comprises a main resonant circuit including as elements a transformer secondary coiland a condenser, atleast one of which is adjustable to tune the coupling system over a range in frequency, a second circuit coupled to said resonant circuit and including a transformer primary coil, said second circuit being resonant at a frequency lower than the lowest frequency within said range, said transformer primary being electrovmagnetically coupled to said main resonant circuit, and said second circuit including 'an electron discharge tube having its output electrodes connected across said output transformer primary.

4. In combination, a thermionic tube having input and output electrodes, a tunable input circuit therefor, a second thermicnic tube having input and output electrodes and a tunable input circuit therefor, the output circuit of said first tube being coupled to the tunable input circuit of said second tube and including a transformer so disposed as to supply energy to the tunable input circuit of the second tube from the output circuit of the rst tube, the primary Winding of said transformer having a natural period below the tuning range of said tunable input circuits whereby the reactance of the output circuit of said rst tube is capacitive over said tuning range, said last mentioned output circuit of said first tube being maintained as close to resonance as possible consistent with stability over the said tuning range.

5. In combination, a thermonic device having filament, grid and plate electrodes, a tunable input circuit connected to said grid and filament electrodes, and an inductance having a natural period below, but close to, the tuning range of said input circuit connected between said' plate and filament electrodes and comprising the primary coil of an inter-tube transformer.

6. In a coupling network adapted to transfer a predetermined range of frequencies between a plurality of thermionic tubes, a transformer having a primary winding the inductance and distributed capacity of which is such as to give it a natural period of vibration below said range, a secondary winding, tuning means for adjusting the period of said secondary winding to frequencies lying within said range, said primary winding being maintained as close to resonance as possible consistent with stability over the said range of frequencies to be transmitted.

7. In combination, a thermionic tube having input and output electrodes, a tunable input circuit therefor, and an output circuit comprising the primary winding of a transformer so disposed as to supply energy to a tunable input circuit of a second thermionic tube, the said primary winding having a maximized capacity whereby it possesses a natural period below, but close to, the tuning range of said input circuits so that the reactance of the output circuit of said first tube is capacitive over the said tuning range.

8. In a radio frequency amplifier, means for preventing self-oscillation throughout a wide band of frequencies to be amplified, consisting of one or more thermionic tubes having grid circuits and plate circuits with a coupling therebetween, means for introducing aA greater amount of capacity in the plate circuits than the amount required to produce resonance -at the lowest frequency of said band of frequencies to be amplified and to which the grid circuit is terminals of said tuned to prevent the control of said grid circuit by said plate circuit, said last mentioned means additionally being designed to maintain said plate circuits as close to resonance with said lowest frequency as possible consistent with stability over the said band of frequencies to secure a high degree of relay amplification.

9. In combination, an amplifier tube having input and output electrodes, a tunable radio frequency input circuit connected between Ithe input electrodes, and an output circuit connected tothe output electrodes, a second amplifier tube,

a tunable radio frequency input circuit connected between the input electrodes of said second,

tube, a' transformer coupling the output circuit of said first tube and the input circuit of said second tube and the primary winding of said transformer being disposed in said output circuit, the said primary winding having a maximized capacity of a magnitude such that the winding is resonant to a frequency sufficiently below the tuning range of said input circuits to prevent the occurrence of oscillations at the lowest frequency to be received. 10. In combination, an amplifier tube having input and output electrodes, a tunable radio frequency input circuit connected between the input electrodes, and an output circuit connected to the output electrodes, a second amplifier tube, a tunable radio frequency input circuit connected between the input electrodes of said second tube, a transformer coupling the output circuit of said first tube and the input circuit of said second tube and the primary winding of said transformer being disposed in said output circuit, the said primary Winding having a capacity of a magnitude such that the winding is resonant to a frequency sufficiently below the tuning range of said input circuits to prevent the occurrence of oscillations at the lowest frequency to be received, and simultaneously to preventthe transfer of energy to said second amplifier tube from increasing at the higher radio frequencies to be received.

11. In combination, an amplifier tube having input and output electrodes, a tunable vradici frequency input lcircuit connected between the input electrodes, an an output circuit connected to the output electrodes, a second amplifier tube, a tunable radio frequency input circuit connected between the input electrodes of said second tube, a transformer coupling the output circuit of said first tube and the input circuit of said second tube and the primary winding of said transformer being disposed in said output circuit, the said primary winding having a small inductance, and a large condenser connected across the winding so that the latter has a maximized capacity of a magnitude such that the winding is 'resonant to a frequency suiciently below the tuning range of said input circuits to prevent the occurrence of oscillations at the lowest frequency to be received.

12. In combination, an amplifier tube having input and output electrodes, a tunable radio frequency input circuit connected between the input electrodes, and an output circuit connected to the output electrodes, a second amplifier tube, a tunable radio frequency input circuit connected between the input electrodes of said second tube, a transformer coupling the output circuit of said first tube and the input circuit of said second tube and the primary winding of said transformer being disposed in said output circuit, the said primary winding having a maximized capacity of a magnitude such that the winding is resonant to a frequency sufficiently below the tuning range of said input circuits to prevent the occurrence of oscillations at the lowest frequency to be received, said primary winding consisting of two portionsonly one of which is magnetically coupled to the secondary winding of said transformer.

13. In combination, an amplifier tube having input and output electrodes, a tunable radio frequency input circuit connected between the input electrodes, and an outputxcircuit connectthe input electrodes of said said input circuits to prevent the occurrence of oscillations at the lowest frequency to be received, -said primary winding consisting of two portions, only one of which is magnetically coupled to the secondary winding of said transformer.

EDWIN A. BEANE.

EDWARD F. ANDREWS. 

